The invention relates generally to laser velocimetry and more specifically concerns a signal processor for measuring the oscillating frequency contained in a laser velocimeter signal burst or bursts.
A laser velocimeter is a non-intrusive instrumentation system that is used to measure the velocity of small particles embedded within a fluid flow field. The transmission optical system divides a single laser beam into two equal-path-length beams and focuses these beams so they cross at a point within the flow field. Since laser light is monochromatic and coherent, a fringe pattern will be formed within the sample volume at the crossover point. As a small particle passes through the fringe pattern, it will scatter light whose intensity will oscillate as the particle passes through the alternating light and dark regions of the fringe pattern. A portion of this scattered light is collected by a receiver which directs the collected light to the photocathode surface of a photomultiplier. The photomultiplier converts the optical signal to an electronic signal. The output electronic signal is composed of a collection of Poisson distributed photo-electrons whose average occurrence rate is proportional to the instantaneous light intensity at the photocathode. As the intensity increases from the photon resolved regime, i.e., one photon per response time of the photomultiplier, the additional photon arrivals within the response time add voltage to the output signal. With sufficient photons, the signal approaches a Gaussian shaped signal burst containing the oscillation frequency. A measurement of the oscillation frequency is multiplied by the distance between adjacent fringes to yield the velocity of the particle.
The primary goal of a signal processing technique is to determine the oscillation frequency contained within a signal burst. In the past there have been three techniques used to determine this oscillation frequency. These three techniques are referred to as the frequency tracking technique, the high-speed burst counter technique and the photon correlator.
The approach used in the frequency tracker technique for the measurement of the oscillation frequency is based on the technology used in an FM radio receiver. The input signal frequency is electronically mixed with a signal from a reference oscillator (usually a voltage controlled oscillator, VCO) to yield a difference frequency located within the bandwidth of an electronic frequency discriminator (frequency to voltage converter). When the reference oscillator frequency is properly set, the average output signal obtained from the discriminator is zero volts and the instantaneous output voltage represents the demodulated FM signal. If the discriminator output is low pass filtered and used as a nulling control signal in the adjustment of the reference oscillator frequency, an automatic frequency control (AFC) circuit is obtained which locks in the radio signal removing the effect of frequency drift. This principle is used in the frequency tracker to follow slow changes in the average signal frequency which result from flow field drift or scanning of the measurement volume within the flow field. This control signal becomes the output signal representing the average flow velocity. The discriminator output represents the instantaneous signal frequency about the average. A weakness of the frequency tracker becomes evident when applied to sparsely seeded gas flow fields. If the duty cycle (percentage of time a signal is present) begins to decrease, the null output from the discriminator (when a signal is not present) decreases the effectiveness of the feedback loop to respond to real changes in the flow velocity due to time weighting of the null signal. If the velocity changes are great enough to leave the bandwidth of the discriminator, e.g., moving the sample volume from a boundary layer to free stream, the signal is lost and cannot be recovered without a full range sweep of the VCO. This bandwidth limit of the discriminator has the second effect of limiting the ability of the frequency tracker to measure flow fields with large turbulence intensities. For example, if the average signal frequency is 20 MHz with a ten percent turbulence intensity, a bandwidth of three standard deviations or twelve MHz (three times .+-.2 MHz), is required for an accurate measurement. Since discriminators typically have a 1.0 MHz bandwidth, the results from the frequency tracker for this example would be considerably in error.
The approach used in the high-speed burst counter technique is much simpler than the tracker: determine when a signal burst is starting, determine when the pedestal removed signal crosses zero volts, determine when the signal crosses zero volts n times later, and output the number of reference clock pulses which occurred during the time for n zero crossings. The technique begins by bandpass filtering the incoming signal to remove the pedestal voltage (high pass section) and increase signal-to-noise by limiting the noise bandwidth (low pass section). The filtered signal then passes through a double threshold/zero detector circuit which converts it to a square wave signal. This signal controls digital logic establishing the gating control signal for circuits which count pulses from the reference clock. Various techniques are then used to validate signals matching the "typical" laser velocimeter signal signature, e.g., 5:8 count comparison. The number of reference clock pulses counted are then output in a digital format to the data acquisition system. The advantages of the high-speed burst counter are: measurement is completed during the signal burst, output only obtained when a measurement has been made, measurements are independent of previous history, and the time of occurrence may be determined. The disadvantages are the effects of signal-to-noise on the determination of the first and n.sup.th zero crossings and quantizing of time by the reference clock yields a residual f'/F (standard deviation of signal frequency divided by the average signal frequency), and serious measurement errors are possible if the filter and threshold settings are chosen improperly.
The operation of the photon correlator is simpler yet. The input signal is autocorrelated for the measurement time and the results sent to the data acquisition system. To increase the speed of the instrument and thus the range of input signal frequencies, the input signal is discriminated by a single comparator to yield a pulse train allowing single bit correlation. This method simplifies the multiplying circuits to a series of logic AND gates. The advantage of this technique is that correlation of the signal will increase the signature of the "average" signal burst while removing the uncorrelated photomultiplier shot noise. The disadvantages of this technique are that only the "average" frequency can be measured, flow turbulence decreases the number of cycles in the correlation function thereby decreasing accuracy, and the discriminator level must be set based on the amplitude of the signal bursts to minimize the effect of noise on the correlation function.
An object of this invention is to provide a signal processing technique that has the ability to follow the average signal frequency variations (automatic frequency control) and maintain a constant signal level (automatic gain control) like the frequency tracker, measure the signal frequency during the signal burst like the high-speed burst counter, and be virtually independent of input signal-to-noise ratio like the photon correlator.
Another object of this invention is to provide a signal processing technique without opeator intervention.
A further object of this invention is to use a non-linear analog-to-digital converter for signal amplitude compression thereby reducing the occurrences of missing cycles during digitization of signal bursts with low visibility.
Yet another object of this invention is to provide a signal integration scheme for determining the capture of randomly occurring signal bursts.
Still another object of this invention is to use digital signal integration for setting the gain of the input signal amplifier that yields a measurement dynamic range greater than previous laser velocimeter signal processors.
A still further object of this invention is to use a digital filter bank to determine the frequency of the measured signal burst.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings.